If you nail the guys dropping down on ropes they don't get a chance to fire at you, but the guys on the balconies and roof will still be taking shots at you. I tend to aim for the guys on the balconies and roof first, then shoot the guys who come down the ropes as they hit the floor. It's one of the hardest battles of the entire game, enjoyable - and satisfying when you beat it (especially on realistic).